The present invention relates to an apparatus and method for arc fault detection and more particularly relates to an apparatus and method for both a stand alone arc fault detector and an arc fault detector combined with a circuit interrupter device.
Circuit breakers, fuses and ground fault circuit interrupters (GFCIs) are commonly used devices for protecting people and property from dangerous electrical faults. Fatalities and loss of property, however, still occur, being caused by electrical faults that go undetected by these protective devices. One such type of electrical fault that typically goes undetected are arc faults. Arcs are potentially dangerous due to the high temperatures contained within them. Thus, they have a high potential of creating damage, mostly through the initiation of fires. An arc, however, will only trip a GFCI if it produces sufficient current leakage to ground. In addition, an arc will trip a breaker only if the current, flowing through the arc, exceeds the trip parameters of the thermal/magnetic mechanism of the breaker. Therefore, an additional type of protection device is needed to detect and interrupt arcs that do not fit these criteria. An arc detector whose output is used to trigger a circuit interrupting mechanism is referred to as an arc fault circuit interrupter (AFCI).
According to the Consumer Product Safety Commission (CPSC) in 1992, it was estimated that xe2x80x9cthere were 41,000 fires involving home electrical wiring systems . . . which resulted in 320 deaths, 1600 injuries and $511 million in property losses.xe2x80x9d The CPSC further stated that xe2x80x9can electrically caused fire may occur if electrical energy is unintentionally converted to thermal energy and if the heat so generated is transferred to a combustible material at such a rate and for such a time as to cause the material to reach its ignition temperature.xe2x80x9d The two main causes of unintentional conversion of electrical energy to heat are excessive current and arcing. Circuit breakers and fuses are currently available to mitigate the results of excessive current, but no commercial system exists to mitigate arcing.
A dangerous condition may develop whenever prolonged arcing exists regardless of whether it involves industrial, commercial or residential power lines. However, mobile homes and especially homes with antiquated wiring systems are particularly vulnerable to fires started due to electrical causes. CPSC studies have shown that the frequency of wiring system fires is disproportionately high in homes over 40 years old.
The causes of arcing are numerous, for example: aged or worn insulation and wiring; mechanical and electrical stress caused by overuse, over currents or lightning strikes; loose connections; and excessive mechanical damage to insulation and wires. Two types of arcing occur in residential and commercial buildings: contact arcing and line arcing. Contact (or series) arcing occurs between two contacts in series with a load. Therefore, the load controls the current flowing in the arc. Line (or parallel) arcing occurs between lines or from a line to ground. Thus, the arc is in parallel with any load present and the source impedance provides the only limit to the current flowing in the arc. It is important for any arc detection system to be able to detect both contact and line arcing and to act appropriately depending upon the severity of the arc.
An example of contact arcing is illustrated in FIG. 1. The conductors 114, 116 comprising the cable 110, are separated and surrounded by an insulator 112. A portion of the conductor 114 is broken, creating a series gap 118 in conductor 114. Under certain conditions, arcing will occur across this gap, producing a large amount of localized heat. The heat generated by the arcing might be sufficient to break down and carbonize the insulation close to the arc 119. If the arc is allowed to continue, enough heat will be generated to start a fire.
A schematic diagram illustrating an example of line arcing is shown in FIG. 2. Cable 120 comprises electrical conductors 124, 126 covered by outer insulation 122 and separated by inner insulation 128. Deterioration or damage to the inner insulation at 121 may cause line fault arcing 123 to occur between the two conductors 124, 126. The inner insulation could have been carbonized by an earlier lightning strike to the wiring system, or it could have been cut by mechanical action such as a metal chair leg cutting into an extension cord.
The potentially devastating results of arcing are widely known and a number of methods of detecting arcs have been developed in the prior art. A large percentage of the prior art refers to detecting the high frequency signals generated on the AC line by arcs. FIG. 3 shows the wide spectrum noise 162 produced on the AC line by an arc. It is superimposed over the AC line voltage 164. An analysis of the arc waveform, using a frequency spectrum analyzer, shows that the overtones and high frequency harmonics contained within the waveform extend well into the GHz range. A graph illustrating the frequency spectrum analysis of the waveform 162 shown in FIG. 3 is shown in FIG. 4.
One major problem associated with any type of arc detection is false tripping. False tripping occurs when an arc detector produces a warning output, or disconnects a section of wiring from the voltage source, when a dangerous arcing condition does not actually exist. The two major causes of false tripping are normal appliance arcing and the inrush currents created by inductive and capacitive appliances. These two situations generate high frequency signals on the power line that are very similar to those generated by dangerous arcing. Thus, to be viable commercial devices, arc detectors must be able to distinguish arcing signals from the signals created by normal appliance use.
A wide range of prior art exists in the field of arc detection. Some of the prior art refers to specialized instances of arcing. For example, U.S. Pat. No. 4,376,243, issued to Renn, et al., teaches a device that operates with DC current. U.S. Pat. No. 4,658,322, issued to Rivera, teaches a device that detects arcing within an enclosed unit of electrical equipment. U.S. Pat. No. 4,878,144, issued to Nebon, teaches a device that detects the light produced by an arc between the contacts of a circuit breaker.
In addition, there are several patents that refer to detecting arcs on AC power lines that disclose various methods of detecting high frequency arcing signals. For example, U.S. Pat. Nos. 5,185,684 and 5,206,596, both issued to Beihoff et al., employ a complex detection means that separately detects the electric field and the magnetic field produced around a wire. U.S. Pat. No. 5,590,012, issued to Dollar, teaches measuring the high frequency current in a shunted path around an inductor placed in the line, which can be the magnetic trip mechanism of a breaker. In a second detection circuit, proposed by Dollar, high frequency voltage signal is extracted from the line via a high pass filter placed in parallel with any load.
Various methods can be found in the prior art to authenticate arcing and to differentiate arcing from other sources of noise. Much of the prior art involves complicated signal processing and analysis. U.S. Pat. No. 5,280,404, issued to Ragsdale, teaches looking for series arcing by converting the arcing signals to pulses and counting the pulses.
In addition, several patents detect arcing by taking the first derivative or second derivative of the detected signal. For example, U.S. Pat. No. 5,224,006, issued to MacKenzie et al., and U.S. Pat. Nos. 5,185,684 and 5,206,596, issued to Beihoff et al, disclose such a device.
Blades uses several methods to detect arcs as disclosed in U.S. Pat. Nos. 5,223,795, 5,432,455 and 5,434,509. The Blades device is based on that fact that detected high frequency noise must include gaps at each zero crossing, i.e., half cycle, of the AC line. To differentiate arcing from other sources of noise, the Blades device measures the randomness and/or wide bandwidth characteristics of the detected high frequency signal. The device taught by U.S. Pat. No. 5,434,509 uses the fast rising edges of arc signals as a detection criterion and detects the short high frequency bursts associated with intermittent arcs.
U.S. Pat. No. 5,561,505, issued to Zuercher et al., discloses a method of detecting arcing by sensing cycle to cycle changes in the AC line current. Differences in samples taken at the same point in the AC cycle are then processed to determine whether arcing is occurring.
The arc fault detection device of the present invention can operate either stand alone or in combination with a circuit interrupting device such as a ground fault circuit interrupter (GFCI). The combination device, known as an arc fault circuit interrupter/ground fault circuit interrupter (AFCI/GFCI), is realized by the addition of extra circuitry to a standard GFCI. An AFCI/GFCI device is a combination arc fault and ground fault detector, having the ability to interrupt the circuit and thereby prevent dangerous arcing and ground fault conditions from harming personnel or property. The term xe2x80x98circuit interrupting devicexe2x80x99 is defined to mean any electrical device used to interrupt current flow to a load and includes, but is not limited to devices such as Ground Fault Circuit Interrupters (GFCIs), Immersion Detection Circuit Interrupters (IDCIs) or Appliance Leakage Circuit Interrupters (ALCIs).
In the AFCI/GFCI circuit of the present invention, an arcing signal is detected on the AC line via two identical pickup coils: a line side coil and a load side coil. The signal from each pickup coil is fed into its own processing circuitry comprising an automatic gain control (AGC) amplifier, a frequency selective network, a perfect rectifier and a time delay peak detector. The output of the peak detector in the line side circuit is fed back to the AGC amplifiers in the load side circuit and vice versa. This unique approach enhances the reliability of arc detection.
The detection of an arc by the device of the present invention is limited to detecting an absolute value of the amplitude of the arc as a result of the electromagnetic generated voltage or current on the power line. The detection comprises ideal rectification of the chaotic waveform. The signal is extracted in a novel manner by utilizing a variable gain controlled (transconductance) amplifier with a compression ratio of at least 40 dB at the input of the signal processing path. A suitable amplifier is one manufactured by Plessey, England. This scheme permits even very large arcs to be detected without overloading the processing circuitry.
A unique aspect of the present invention is that it is capable of distinguishing between arc faults on the line and load sides of the device. Depending on the location of the arc fault, i.e., line side or load side, AC power is either disconnected to the load or an audible or visual annunciator is activated. Once processed, the peak amplitudes of the two sense signals, i.e., line side and load side sense signals, are compared via two comparators. If the signal generated by the line side circuit is greater than the signal generated by the load side circuit, the output causes an audible or visual indication to be generated. On the other hand, if the arc signal generated by the line side circuit is less than the signal generated by the load side circuit, the interrupting mechanism of the GFCI is activated and the load is disconnected from the AC line. Thus, arcs detected occurring on the load side of the device cause the device to disconnect the AC line from the load.
The use of two different sensing circuits generating separate line and load side signals provides the following three advantages.
1. If an arc occurs on the load side of the AFCI/GFCI, the device will trip and the arc will be extinguished. However, equipment located up stream from the device can still function since AC power to them is not interrupted.
2. Locating the position of a fault is simplified when several AFCI/GFCI devices are used on the same branch circuit, even without any communication of the occurrence of the fault to a central location.
3. Indicating the presence of arcing on the line side of the AFCI/GFCI permits the detection of a problem between the circuit breaker or transformer and the device, while preventing false tripping from disturbances in the utility distribution system.
The arc detector of the present invention can be implemented as a standalone device or can be implemented in combination with an existing electrical device such as a GFCI. A feature of the arc detector of the present invention is that it combines an arc detector, i.e., arc fault circuit interrupter (AFCI), with a circuit interrupting device, such as a ground fault circuit interrupter (GFCI), to create an AFCI/GFCI multipurpose device. Such a device has the ability to interrupt the AC power and thereby prevent a dangerous arcing or ground fault condition from harming personnel or property. Note that existing GFCIs can detect an arc fault if the arc generated ground fault current from either phase or neutral to ground. However, the AFCI dedicated circuitry functions to detect both series arcs and parallel arcs that do not happen to generate leakage current to ground. The novel use of common circuit elements provides high noise immunity for the arc detector and thus helps to prevent false tripping of the device.
The arc detection circuitry can be placed onboard the same silicon chip typically used in today""s GFCI. Indeed, some of the pins of the currently utilized GFCI integrated circuit can be converted for multifunction operation. The AFCI can be powered from the same power supply that provides power to the GFCI. This combined approach results in reduced manufacturing costs. The mechanical parts of the GFCI device such as the trip relay and the mechanical contact closure mechanisms now serve dual purposes. In addition, adding AFCI circuitry to an existing GFCI is a logical enhancement of present day GFCIs since a GFCI can detect arcing in certain situations including any condition whereby an arc produces leakage current to ground.
The arc detector also incorporates an automatic bypass timer controls the AC line disconnect function in order to permit normally safe arcing. Rather than include an on/off fixed switch which would function to completely enable or disable the arc detector, the present invention incorporates a logical switch. This logic driven switch provides a user with the option of disabling the arc detector for as long as the switch is off or disabling the arc detector temporarily while arcing appliances are in use. This permits the use of appliances that normally generate high amounts of arcing that would otherwise cause the arc detector to trip. When the arc detector is temporarily disabled, it automatically return to the enabled state after the appliance has been disconnected. This scheme has the advantage that the device cannot accidentally be permanently disabled by the user. An important feature of this scheme is that the arcing appliance can be turned on and off within the given time period without tripping the arc detector. Note that the ground fault detection capability of the device is never disabled, so the user is always protected from ground faults.
Today, AC power lines are not only used for supplying AC line current but they are also used as a media for communications as in Leviton Manufacturing""s CCS line of power line carrier devices, CEBus compatible devices, power line carrier based intercoms, TV signal transmission/reception equipment, telephone communication devices, etc. The arc detector of the present invention incorporates a filter circuit having a sharp cut off slope of approximately 500 KHz which permits the detection of arc faults while communications over the AC power lines is occurring. The filter circuit functions to remove frequencies below 500 KHz thus preventing false tripping due to the various communication signals potentially present on the AC line while permitting the arc fault device to communicate with other devices using power line carrier communications. On the other end of the frequency spectrum, although arcing generates frequencies into the GHz range, for simplicity, efficiency and cost effectiveness, the arc detector of the present invention limits detection of high frequency signals to approximately 20 MHz.
Further, the arc detector includes circuitry to transmit messages using any suitable communication means pinpointing the location of arc fault. For example, such communication means may comprise any power line carrier, RF, twisted pair or IR communications technology. An example of power line carrier communications include Lon Works and CEBus communications systems. By way of example only, the present invention incorporates a communications circuit, which in utilizes a power line carrier signal such as generated by the CCS product line manufactured by Leviton Manufacturing, Little Neck, N.Y. Using well known power line carrier techniques the arc detector can communicate with another device such as a monitoring station. Each arc detector would have a unique address. A relationship is then established between the address assigned to the arc detector and its location. When an arc fault is detected a signal is sent to a monitoring station which alerts personnel of not only the occurrence of the arc fault but also its location. This is helpful especially if the AFCI/GFCI device is installed in a remote location. This feature has applicability in industrial and commercial locations where central arc fault supervision over a complex AC electrical wiring system is needed.